U.S. patent application number 11/211278 was filed with the patent office on 2006-03-30 for recovering natural gas liquids from lng using vacuum distillation.
This patent application is currently assigned to Chevron U.S.A. Inc.. Invention is credited to Michael Monroe McCoy.
Application Number | 20060065015 11/211278 |
Document ID | / |
Family ID | 36097488 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060065015 |
Kind Code |
A1 |
McCoy; Michael Monroe |
March 30, 2006 |
Recovering natural gas liquids from LNG using vacuum
distillation
Abstract
A process for recovering liquefied C.sub.3 compounds from a
natural gas stream containing C.sub.3 compounds which comprises (a)
chilling the natural gas stream under conditions sufficient to
liquefy the natural gas, whereby a mixture comprising liquid
natural gas (LNG) and liquid C.sub.3 compounds is formed; (b)
fractionating the mixture comprising liquid natural gas (LNG) and
liquid C.sub.3 compounds in a separation zone under sub-atmospheric
pressure and under conditions predetermined to vaporize a
significant portion of the natural gas present while retaining the
C.sub.3 compounds as a liquid fraction; and (c) recovering
separately from the separation zone natural gas and liquid C.sub.3
compounds.
Inventors: |
McCoy; Michael Monroe;
(Sugar Land, TX) |
Correspondence
Address: |
CHEVRON TEXACO CORPORATION
P.O. BOX 6006
SAN RAMON
CA
94583-0806
US
|
Assignee: |
Chevron U.S.A. Inc.
|
Family ID: |
36097488 |
Appl. No.: |
11/211278 |
Filed: |
August 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60614664 |
Sep 29, 2004 |
|
|
|
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 2210/62 20130101;
C07C 7/09 20130101; C07C 11/06 20130101; C07C 9/08 20130101; C07C
7/09 20130101; F25J 3/0247 20130101; F25J 2270/904 20130101; F25J
3/0242 20130101; F25J 3/0209 20130101; F25J 2245/90 20130101; F25J
2200/72 20130101; F25J 3/0233 20130101; F25J 1/0231 20130101; C07C
7/09 20130101; F25J 1/0022 20130101; F25J 2290/12 20130101; F25J
2200/02 20130101; F25J 2215/64 20130101; B01D 3/146 20130101; F25J
2215/66 20130101; F25J 2290/62 20130101 |
Class at
Publication: |
062/620 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A process for recovering liquefied C.sub.3 compounds from a
natural gas stream containing C.sub.3 compounds which comprises:
(a) chilling the natural gas stream under conditions sufficient to
liquefy the natural gas, whereby a mixture comprising liquid
natural gas (LNG) and liquid C.sub.3 compounds is formed; (b)
fractionating the mixture comprising liquid natural gas (LNG) and
liquid C.sub.3 compounds in a separation zone under sub-atmospheric
pressure and under conditions predetermined to vaporize a
significant portion of the natural gas present while retaining the
C.sub.3 compounds as a liquid fraction; and (c) recovering
separately from the separation zone natural gas and liquid C.sub.3
compounds.
2. The process of claim 1 wherein the separation zone is maintained
at a pressure of less than 12 psia and at a bottoms temperature
within the range of from about 0.degree. F. to about 200.degree.
F.
3. The process of claim 2 wherein the bottoms temperature in the
separation zone is maintained at a temperature within the range
from about 20.degree. F. to about 150.degree. F.
4. The process of claim 1 wherein the natural gas stream also
contains C.sub.4 compounds and a mixture comprising liquid
C.sub.3-C.sub.4 compounds are recovered from the separation
zone.
5. The process of claim 4 including the additional steps of: (d)
introducing the mixture comprising liquid C.sub.3-C.sub.4 compounds
into a second separation zone under sub-atmospheric pressure and
under conditions predetermined to vaporize a significant portion of
the C.sub.3 compounds present while retaining the C.sub.4 compounds
as a liquid fraction; and (e) recovering separately from the second
separation zone C.sub.3 compounds and liquid C.sub.4 compounds.
6. The process of claim 5 wherein the second separation zone is
maintained at a pressure of less than 12 psia and at a bottoms
temperature within the range of from about 20.degree. F. to about
250.degree. F.
7. The process of claim 6 wherein bottoms temperature in the second
separation zone is maintained within the range of from about
40.degree. F. to about 150.degree. F.
8. The process of claim 5 wherein the natural gas stream also
contains C.sub.5+ compounds and a mixture comprising liquid C.sub.4
and C.sub.5+ compounds are recovered from the second separation
zone.
9. The process of claim 8 including the additional steps of: (f)
introducing the mixture comprising liquid C.sub.4 and C.sub.5+
compounds into a third separation zone under sub-atmospheric
pressure and under conditions predetermined to vaporize a
significant portion of the C.sub.4 compounds present while
retaining the C.sub.5+ compounds as a liquid fraction; and (g)
recovering separately from the third separation zone C.sub.4 and
C.sub.5+ compounds.
10. The process of claim 9 wherein the third separation zone is
maintained at a pressure of less than 12 psia and at a bottoms
temperature within the range of from about -75.degree. F. to about
75.degree. F.
11. The process of claim 10 wherein the bottoms temperature in the
third separation zone is maintained within the range of from about
-50.degree. F. to about 50.degree. F.
12. A process for recovering liquefied C.sub.3+ compounds from a
natural gas stream containing C.sub.3, C.sub.4, and C.sub.5+
compounds which comprises: (a) chilling the natural gas stream
under conditions sufficient to liquefy the natural gas, whereby a
mixture comprising liquid natural gas (LNG) and liquid C.sub.3+
compounds is formed; (b) fractionating the mixture comprising
liquid natural gas (LNG) and liquid C.sub.3+ compounds in a
separation zone under sub-atmospheric pressure and under conditions
predetermined to vaporize a significant portion of the natural gas
present while retaining the C.sub.3+ compounds as a liquid
fraction; (c) recovering separately from the separation zone
natural gas and a mixture comprising liquid C.sub.3+ compounds; (d)
introducing the mixture comprising liquid C.sub.3+ compounds into a
second separation zone under sub-atmospheric pressure and under
conditions predetermined to vaporize a significant portion of the
C.sub.3 and C.sub.4 compounds present while retaining the C.sub.5+
compounds as a liquid fraction; (e) recovering separately from the
second separation zone liquid C.sub.5+ compounds and a mixture
comprising C.sub.3-C.sub.4 compounds.
13. The process of claim 12 wherein the second separation zone is
maintained at a pressure of less than 12 psia and at a bottoms
temperature within the range of from about -75.degree. F. to about
75.degree. F.
14. The process of claim 13 wherein the bottoms temperature in the
second separation zone is maintained within the range of from about
-50.degree. F. to about 50.degree. F.
15. The process of claim 12 including the additional steps of: (f)
introducing the mixture comprising the C.sub.3-C.sub.4 compounds
into a third separation zone under sub-atmospheric pressure and
under conditions predetermined to vaporize a significant portion of
the C.sub.3 compounds present while retaining the C.sub.4 compounds
as a liquid fraction; and (g) recovering separately from the third
separation zone C.sub.3 compounds and liquid C.sub.4 compounds.
16. The process of claim 15 wherein the third separation zone is
maintained at a pressure of less than 12 psia and at a bottoms
temperature within the range of from about 20.degree. F. to about
250.degree. F.
17. The process of claim 16 wherein bottoms temperature in the
third separation zone is maintained within the range of from about
40.degree. F. to about 150.degree. F.
18. A process for producing liquefied C.sub.3 product and liquefied
C.sub.4 product comprising fractionating a liquid comprising
C.sub.3 compounds and C.sub.4 compounds in a separation zone at
pressures maintained at sub-atmospheric pressure and separately
recovering at least liquefied C.sub.3 product and liquefied C.sub.4
product.
19. The process of claim 18 wherein the pressure in the separation
zone is maintained below 12 psia.
20. The process of claim 18 wherein boil off gas from a C.sub.3
product storage facility is added to the separation zone and
recovered as part of the liquefied C.sub.3 product.
21. The process of claim 18 wherein boil off gas from a C.sub.4
product storage facility is added to the separation zone and
recovered as part of the liquefied C.sub.4 product.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application No. 60/614,664 filed on Sep. 29, 2004, the entire
contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is directed to a process for
recovering natural gas liquids from liquefied natural gas.
BACKGROUND OF THE INVENTION
[0003] Liquefied natural gas (LNG) is principally liquid methane
with smaller amounts of C.sub.2+ hydrocarbons also present. LNG is
prepared by chilling a raw natural gas stream to a temperature and
at a pressure sufficient to cause at least a portion of the methane
in the raw gas to condense as a liquid. The natural gas stream from
which the LNG is made may be recovered from any process which
generates light hydrocarbon gases. However, generally the raw
natural gas from which LNG is prepared is recovered from a crude
oil or gas well.
[0004] Raw natural gas is a mixture of various hydrocarbon gases,
including C.sub.2- hydrocarbons, and heavier C.sub.3 and C.sub.4
petroleum gases. "Wet" gas also comprises varying amounts of
C.sub.5+ hydrocarbons, while "dry" gas comprises little or no
C.sub.5+ hydrocarbons. As used herein, C.sub.1 represents a
hydrocarbonaceous compound having one carbon atom per molecule,
C.sub.2 contains two carbon atoms per molecule, etc.
C.sub.3-C.sub.4 represents a hydrocarbonaceous material comprising
compounds having three carbon atoms per molecule and/or compounds
having four carbon atoms per molecule. C.sub.3+ compounds
represents compounds having three or more carbon atoms per
molecule. C.sub.5+ represents compounds having five or more carbon
atoms per molecule. Methane is a representative example of a
C.sub.1 compound. Ethane, ethylene, and mixtures thereof are
representative examples of C.sub.2 compounds. Propane, propene,
butane, butenes and mixtures thereof are representative examples of
C.sub.3-C.sub.4 compounds. Pentanes, pentenes, hexanes, hexenes and
comparable higher molecular weight species, and their mixtures, are
representative of C.sub.5+ compounds.
[0005] The process of liquefying natural gas involves chilling the
raw natural gas, either at atmospheric or super-atmospheric
pressure, until the methane and ethane condense as liquids. On
account of their higher molecular weights and lower dew points, any
C.sub.3+ vapors contained in the raw natural gas condense prior to
the condensation of the C, and C.sub.2 compounds, forming a liquid
product termed "natural gas liquids" which may be abbreviated as
"NGL". Each of the components which condense during the preparation
of LNG have important commercial value. C.sub.1 and C.sub.2
compounds are the major components of LNG and are valuable both as
fuel and as feedstock for preparing commercially valuable products.
Liquefied petroleum gas (LPG), comprising principally C.sub.3 and
C.sub.4 hydrocarbons, is useful as a refrigerant in the
liquefaction process. LPG also may serve as a fuel in the LNG
liquefaction process or as transportation or heating fuel. The
C.sub.5+ condensate recovered from the raw natural gases is
valuable as a blending component for fuels, particularly for
transportation fuels. Therefore, it is important that the liquefied
C.sub.5+ condensate and the C.sub.3-C.sub.4 LPG be recovered
separately from the LNG. The present invention is directed to an
efficient process for recovering and storing separate LPG streams
in the process of preparing LNG.
[0006] As used in this disclosure the words "comprises" or
"comprising" are intended as open-ended transitions meaning the
inclusion of the named elements, but not necessarily excluding
other unnamed elements. The phrases "consists essentially of" or
"consisting essentially of" are intended to mean the exclusion of
other elements of any essential significance to the composition.
The phrases "consisting of" or "consists of" are intended as a
transition meaning the exclusion of all but the recited elements
with the exception of only minor traces of impurities.
SUMMARY OF THE INVENTION
[0007] The present invention relates to a method for separating
materials which are included in raw natural gas, particularly the
materials which are recovered as natural gas liquids during the
preparation of liquefied natural gas. When the process is used
primarily to recover C.sub.3 compounds from the raw natural gas,
the invention may be broadly described as a process for recovering
liquefied C.sub.3 compounds from a natural gas stream containing
C.sub.3 compounds which comprises (a) chilling the natural gas
stream under conditions sufficient to liquefy the natural gas,
whereby a mixture comprising liquid natural gas (LNG) and liquid
C.sub.3 compounds is formed; (b) fractionating the mixture
comprising liquid natural gas (LNG) and liquid C.sub.3 compounds in
a separation zone under sub-atmospheric pressure and under
conditions predetermined to vaporize a significant portion of the
natural gas present while retaining the C.sub.3 compounds as a
liquid fraction; and (c) recovering separately from the separation
zone natural gas and liquid C.sub.3 compounds. If C.sub.4+
compounds are also present additional separations may be used to
recover these products as well. In this instance, the additional
separation steps may be described as (d) introducing the mixture
comprising liquid C.sub.3-C.sub.4 compounds into a second
separation zone under sub-atmospheric pressure and under conditions
predetermined to vaporize a significant portion of the C.sub.3
compounds present while retaining the C.sub.4 compounds as a liquid
fraction; and (e) recovering separately from the second separation
zone C.sub.3 compounds and liquid C.sub.4 compounds.
[0008] In the event that the raw natural gas also contains
recoverable amounts of C.sub.5+ compounds the invention may be
described as a process for recovering liquefied C.sub.3+ compounds
from a natural gas stream containing C.sub.3, C.sub.4, and C.sub.5+
compounds which comprises (a) chilling the natural gas stream under
conditions sufficient to liquefy the natural gas, whereby a mixture
comprising liquid natural gas (LNG) and liquid C.sub.3+ compounds
is formed; (b) fractionating the mixture comprising liquid natural
gas (LNG) and liquid C.sub.3+ compounds in a separation zone under
sub-atmospheric pressure and under conditions predetermined to
vaporize a significant portion of the natural gas present while
retaining the C.sub.3+ compounds as a liquid fraction; (c)
recovering separately from the separation zone natural gas and a
mixture comprising liquid C.sub.3+ compounds; (d) introducing the
mixture comprising liquid C.sub.3+ compounds into a second
separation zone under sub-atmospheric pressure and under conditions
predetermined to vaporize a significant portion of the C.sub.3 and
C.sub.4 compounds present while retaining the C.sub.5+ compounds as
a liquid fraction; (e) recovering separately from the second
separation zone liquid C.sub.5+ compounds and a mixture comprising
C.sub.3-C.sub.4 compounds. The C.sub.3 compounds and C.sub.4
compounds may be recovered separately by including the additional
steps of (f) introducing the mixture comprising the C.sub.3-C.sub.4
compounds into a third separation zone under sub-atmospheric
pressure and under conditions predetermined to vaporize a
significant portion of the C.sub.3 compounds present while
retaining the C.sub.4 compounds as a liquid fraction; and (g)
recovering separately from the third separation zone C.sub.3
compounds and liquid C.sub.4 compounds.
[0009] Finally, the process may be used as a process for producing
a liquefied C.sub.3 material and a liquefied C.sub.4 material from
a mixture comprising C.sub.3 and C.sub.4 compounds. Accordingly the
process may be described as a process for producing liquefied
C.sub.3 product and liquefied C.sub.4 product comprising
fractionating a liquid comprising C.sub.3 compounds and C.sub.4
compounds in a separation zone at pressures under sub-atmospheric
pressure and separately recovering at least liquefied C.sub.3
product and liquefied C.sub.4 product. As will be explained in
greater detail below, this embodiment of the process may be used to
recover boil off gas from C.sub.3 and C.sub.4 storage
facilities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 represents a flow diagram illustrating a typical
scheme for recovering NGL from raw natural gas.
[0011] FIG. 2 is a schematic diagram of the LPG recovery section
shown in FIG. 1 which illustrates one embodiment of the present
invention.
[0012] FIG. 3 is a schematic diagram illustrating an embodiment of
the invention in which boil off gas from the C.sub.3 and C.sub.4
storage facilities is recovered.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In the process of the invention, liquefied C.sub.3 compounds
and liquefied C.sub.4 compounds are produced by separating a
natural gas liquid at sub-atmospheric pressure in a series of
fractionation zones. As used herein, separation, distillation or
fractionation at sub-atmospheric pressure means distillation at a
pressure less than the ambient pressure, and typically less than 15
psia (absolute pressure in pounds per square inch). It may also be
termed "vacuum" distillation. Conventional methods for making these
separations include fractionation at elevated temperatures and
under pressure. In contrast, in the present process the
distillations are performed at low temperatures and at pressures
below 15 psia (i.e., at sub-atmospheric pressure), and most
preferably at pressures below 12 psia. In the practice of the
invention, costs involved in heating and pressurizing the light
hydrocarbon streams are significantly reduced. In addition, LPG
products, including liquefied propane and liquefied butane, can be
recovered from the fractionation process without requiring
significant additional condensing and pressurization beyond that
required for pumping the fluids through the liquefaction
process.
[0014] In one embodiment of the invention in which C.sub.3,
C.sub.4, and C.sub.5+ compounds are separately recovered from wet
raw natural gas, the process utilizes three separations zones
generally referred to in the art as a deethanizer, debutanizer, and
depropanizer, respectively. The deethanizer which is intended for
the separation of C.sub.2- compounds from C.sub.3+ compounds is
generally operated at a pressure of less than 12 psia and at a
bottoms temperature within the range of from about 0.degree. F.
(about -17.8.degree. C.) to about 200.degree. F. (about
93.3.degree. C.). Preferably, the bottoms temperature of the
deethanizer is maintained within the range of from about 20.degree.
F. (about -6.67.degree. C.) to about 150.degree. F. (about
65.6.degree. C.). The debutanizer which is intended for the
recovery of C.sub.3 compounds from C.sub.4+ compounds is generally
operated at a pressure of less than 12 psia and at a bottoms
temperature within the range of from about 20.degree. F. (about
-6.67.degree. C.) to about 250.degree. F. (about 121.degree. C.).
Preferably, the bottoms temperature of the debutanizer is
maintained within the range of from about 40.degree. F. (about
4.44) to about 150.degree. F. (about 65.6.degree. C.). The
depropanizer which is intended for the recovery of C.sub.4
compounds from C.sub.5+ compounds is generally operated at a
pressure of less than 12 psia and at a bottoms temperature within
the range of from about -75.degree. F. (about -59.4.degree. C.) to
about 75.degree. F. (about 23.9.degree. C.). Preferably, the
bottoms temperature of the depropanizer is maintained within the
range of from about -50.degree. F. (about -45.6.degree. C.) to
about 50.degree. F. (about 10.degree. C.).
[0015] The present invention will be more clearly understood by
reference to the drawings. FIG. 1 represents a typical process
scheme for recovering natural gas liquids (NGL) from raw natural
gas. In FIG. 1, the raw natural gas stream (2) from which LNG is
made is recovered from a production well, either alone or in
combination with heavier crude products. The raw natural gas stream
typically comprises methane, C.sub.2-C.sub.4 hydrocarbons, and
generally lesser amounts of C.sub.5+ condensate. The raw natural
gas stream may contain C.sub.5+ condensate at a concentration
within a wide range, and, if the C.sub.5+ hydrocarbons in the
natural gas stream are present in recoverable amounts, the gas is
referred to as "wet" natural gas. Wet natural gas will typically
contain up to 20% by volume C.sub.5+ condensate (e.g., 0.5%-20%).
"Dry" natural gas will contain virtually no C.sub.5+ condensate.
The stream may also contain contaminants such as water, carbon
dioxide, hydrogen sulfide, nitrogen, dirt, iron sulfide, wax, crude
oil, diamondoids, mercury and the like. These contaminants are
generally undesirable in the liquefied LNG and NGL products.
Contaminants which condense as separate liquid or solid phases
during chilling cause problems during the refrigeration steps, and
are necessarily removed. Acid contaminants which may lead to
corrosion of the refrigeration materials are also desirably
removed. These contaminants may be removed by conventional means
(4) which are well known in the art. After the natural gas stream
is cleaned to remove contaminants, it is sent by line 6 to be
chilled in a 1.sup.st refrigeration zone (8), which comprises one
or more refrigeration cycles. Example coolants used in 1.sup.st
refrigeration zone (8) include LNG, LPG or mixtures thereof. The
chilling process produces a natural gas liquid stream (10) and
often a separate C.sub.5+ condensate stream (12).
[0016] As shown in FIG. 1, the C.sub.5+ condensate stream (12)
removed from the 1.sup.st refrigeration cycle may optionally be
sent by line 14 to the LPG separation zone (16) for removing any
C.sub.4- components (i.e., C.sub.4 and lighter) which are contained
in it.
[0017] Natural gas liquids (10) from the 1.sup.st refrigeration
zone (8) are also passed to an LPG separation zone (16) for the
isolation and separate recovery of liquid C.sub.3 compounds (18)
and liquid C.sub.4 compounds (20). The C.sub.3 and C.sub.4
hydrocarbon products are sent to storage vessels (22) and (24),
respectively. The C.sub.3 compounds in stream (18) and in tank (22)
comprises liquid C.sub.3 hydrocarbons which are primarily propane.
There will generally be amounts of both C.sub.3H.sub.8 and
C.sub.3H.sub.6 hydrocarbons in the liquid C.sub.3 product, the
ratio of the two species ranging from 100% C.sub.3H.sub.8 to 100%
C.sub.3H.sub.6. However, C.sub.3H.sub.8 generally will be the
predominant hydrocarbon. There may also be small amounts of
contaminants in the liquid C.sub.3 product, including some C.sub.2-
materials and some C.sub.4+ materials. The same is true for the
C.sub.4 product stream (20) which is stored in tank (24). There
will generally be amounts of both C.sub.4H.sub.1O and
C.sub.4H.sub.8 hydrocarbons in the liquid C.sub.4 product, the
ratio of the two species ranging from 100% C.sub.4H.sub.10 to 100%
C.sub.4H.sub.8. Generally, C.sub.4H.sub.10 will be the predominant
hydrocarbon. There may also be small amounts of contaminants in the
liquid C.sub.4 product, including some C.sub.3- materials and some
C.sub.5+ materials. A fuel gas stream (26) which is also recovered
from the LPG separation zone (16) is combined with natural gas
stream (28) from 1.sup.st refrigeration zone (8) for additional
cooling in the 2.sup.nd refrigeration zone (30). LNG is recovered
as a liquid stream (32) from the 2.sup.nd refrigeration zone for
storage in storage vessel 34. In one embodiment of the process, LNG
stored in 34 and C.sub.3 product and C.sub.4 product respectively
stored in 22 and 24 are maintained at nominally atmospheric
pressure, the actual pressure being slightly higher than ambient
pressure to account for the vapors which are being generated by the
evaporating liquids and which are being vented from the storage
vessels. The two C.sub.5+ condensate streams (12) and (36), if
present, may be combined or used separately in downstream
processing, as fuel, as a petrochemical feedstock, and the
like.
[0018] The present invention is concerned primarily with the design
and operation of the LPG separation zone (16) shown in FIG. 1. In a
typical conventional method of recovering the propane and butane
(LPG) and the crude condensate (a C.sub.5+ stream of hydrocarbons)
from an LNG facility a series of fractionation columns are employed
which have operating pressures in the 120 psig to 230 psig range
and operating temperatures in the 150.degree. F. (65.6.degree. C.)
to 330.degree. F. range (166.degree. C.), depending on the column.
In an LNG facility, operating at these temperatures and pressures
requires a large amount of energy to heat the LPG and condensate
stream from less than -100.degree. F. (-18.6.degree. C.) to the
200.degree. F. (93.3.degree. C.) range. Furthermore, LNG plants,
unless they have a heat recovery system, do not produce a
sufficiently hot stream that could be used as a heating medium in
the column reboilers and, therefore, it is necessary to install a
special process heater to provide the heat input for the reboilers
on the columns. Also, if the project specifics mandate atmospheric
storage, the LPG streams must be cooled back down to about
-50.degree. F. (-45.6.degree. C.) for storage which again requires
a large amount of energy. Table 1 lists typical operating
conditions for the fractionation columns in a conventional LPG
separation section. TABLE-US-00001 TABLE 1 Typical Operating
Conditions for Separation of Light Petroleum Fractions in a
Conventional LPG Separation Process Temp. .degree. F. Press. psig
Deethanizer Overhead 115.degree. F. 200 psig Bottoms 290.degree. F.
Debutanizer Overhead 150.degree. F. 120 psig Bottoms 330.degree. F.
Depropanizer Overhead 120.degree. F. 230 psig Bottoms 220.degree.
F.
LPG separations under the operating conditions shown in Table 1 are
suitable for a conventional refinery system, in which sufficient
high temperature streams are available for heating the reboiler
exchangers to the high bottoms temperatures typical of the
conventional process by using steam or other readily available
streams as the heating medium.
[0019] FIG. 2 illustrates one embodiment of the process of the
invention, showing the separation and recovery of liquefied C.sub.3
product and liquefied C.sub.4 product as separate streams in a
multiple fractionation process shown generally in FIG. 1 as LPG
separation zone (16). Natural gas liquid (10) from the 1.sup.st
refrigeration zone is prepared in heat exchanger (110) prior to
separation in 1.sup.st fractionation zone (120). Depending on the
temperature of the natural gas liquid (10), this stream may be
either cooled or heated in heat exchanger (110). Normally, heating
will be required. Methods of heating fluid streams are well-known
in the art and should not require further explanation here. The
natural gas liquid (10) is fractionated in the 1.sup.st
fractionation zone (120) to remove light materials, primarily
methane and ethane. The fractionation is conducted at
sub-atmospheric pressure and at a temperature selected to maintain
an acceptable separation efficiency. Methods for maintaining a
vacuum during distillation are well known, and include, for
example, use of an eductor or a vacuum pump. Table 2 broadly lists
operating conditions which are useful in the present process, and
includes two exemplary cases, a lower temperature/lower pressure
case and a higher temperature/relatively higher pressure case.
Typical operating conditions for each fractionation column are
shown in Table 3. In all cases, fractionation pressure in each
fractionation zone is maintained in the sub-atmospheric range.
[0020] To maintain a suitable temperature in the bottom of 1.sup.st
fractionation zone (120), reboiler (130) serves to heat the recycle
liquid (128). Such a reboiler is easily designed by one skilled in
the art, and a detailed description of this heater is not required
here. Any suitable heating medium, e.g. water, may be used, so long
as the temperature of the heating medium is higher than the desired
temperature of the recycle liquid.
[0021] A portion of overhead vapor product (122) is cooled and
condensed in exchanger (150). The cooled and at least partially
liquefied stream is stored in reflux drum (160) and then returned
to the column via pump (170). The cooling medium used to cool the
reflux stream (122) will be any suitable liquid which is at a lower
temperature than the reflux fluid. When the present process is
included within a process for preparing LNG from a raw natural gas
stream, the cooling medium will generally be selected from one of
LNG, LPG or a mixture thereof. Liquid methane (or LNG) is
particularly suited for this use, both because of its low
temperature, and because the overall LNG process is ideally suited
for re-condensing a methane vapor which is generated when liquefied
methane is used in cooling exchanger (150). Overhead product (26)
from the 1.sup.st fractionation zone (120), also termed a
deethanizer, is recovered as fuel gas or is passed to the 2.sup.nd
refrigeration zone (30), which is illustrated in FIG. 1, for
conversion to LNG.
[0022] Bottoms product from fractionation zone (120) is collected
in line 126 and sent to downstream processing for removal of any
remaining C.sub.5+ condensate and for separation of individual
liquid propane and liquid butane streams.
[0023] In the exemplary embodiment of FIG. 2, stream (126) is
passed via pump (140) to the 2.sup.nd fractionation zone (180),
also referred to as a debutanizer, for removing any C.sub.5+
condensate 36. Fractionation zone (180) is also maintained at
sub-atmospheric pressure. Recycle liquid (188) at the bottom of
fractionation zone (180) is maintained at a temperature selected to
achieve a desirable separation of the C.sub.5+ condensate collected
in line 36. Any suitable heating medium, e.g. water, may be used to
provide heat in the reboiler (190). Overhead vapor product (182)
from fractionation zone (180) is cooled sufficiently in condenser
(200) to condense at least a portion of the stream into the liquid
phase. Example coolants include LNG, LPG or mixtures thereof. The
cooled stream is collected in reflux drum (210) and then passed via
pump (220) back to the column as reflux. A portion of the liquid
overhead product is removed as stream (184) from the reflux stream
and is passed to 3.sup.rd fractionation zone (230) for separation
of a liquid propane stream (18) and a liquid butane stream (20). To
facilitate the separation, a portion of the liquid at the bottom of
fractionation zone (230) is removed as recycle liquid (238), heated
in heater (240) and returned as recycle liquid to the column. As in
the other columns, heater (240) may include a suitable heating
medium, such as natural gas liquid stream (10), an LPG stream or
the exhaust from a fin fan exchanger.
[0024] In the particular embodiment shown in FIG. 2, liquid butane
(20) is removed as a liquid bottoms product from fractionation zone
(230). Liquid propane (18) is removed as an overhead product from
the reflux loop of fractionation zone (230). Overhead vapor product
(232) is cooled and at least partially condensed in condenser
(250), collected in reflux drum (260) and returned as liquid reflux
to the column. A portion of the reflux stream is removed as liquid
propane (18). Cooling of the overhead stream in condenser (250)
generally employs a liquid which is at a temperature lower 1 than
the desired temperature of the reflux. Example coolants include
LNG, LPG or a mixture thereof.
[0025] It will be seen that in the embodiment illustrated in FIG. 2
that the C.sub.5+ condensate is first removed and recovered from
2.sup.nd fractionation zone (180) (also termed a debutanizer), and
an overhead stream (184) is passed from the 2.sup.nd fractionation
zone to a 3.sup.rd fractionation zone (230), also termed a
depropanizer, from which separate liquefied C.sub.3 material (18)
and liquefied C.sub.4 material (20) are recovered. In the
embodiment of the invention shown in FIG. 2, therefore, the process
comprises multiple fractionation zones, including a deethanizer, a
debutanizer and a depropanizer in that order. In a separate
embodiment of the invention, the order may be altered, such that
the deethanizer is followed by a depropanizer followed by a
debutanizer. When the depropanizer precedes the debutanizer, a
liquid propane product is recovered by fractionation of the bottoms
stream (126) in a depropanizer, and a following fractionation step
separates the liquid bottoms product from the depropanizer into at
least a liquid butane stream and a bottoms C.sub.5+ condensate
stream in a debutanizer. The order of separations of the
depropanizer and debutanizer depends, at least in part, on the
relative amounts of propane, butane and C.sub.5+ condensate in the
natural gas liquid. Minimum energy requirements are the primary
objective in selecting the order of separations. TABLE-US-00002
TABLE 2 Operating Ranges for Fractionation Columns Shown in FIG. 2
Broad Range Low Pressure High Pressure Temp. .degree. F. Press.
psia Temp. .degree. F. Press. psia Temp. .degree. F. Press. psia
Deethanizer (120) Overhead (122) -150 to 0 <1 atm -100 3 -50 8
Bottoms (126) 0 to 200 60 120 Debutanizer (180) Overhead(182) -100
to 50 <1 atm -50 3 0 8 Bottoms (186) 20 to 250 70 130
Depropanizer (230) Overhead (232) -150 to 0 <1 atm -100 3 -50 12
Bottoms (236) -75 to 75 130 10
[0026] Typical operations conditions for each fractionation column
shown in FIG. 2 are shown in Table 3, below. Temperatures are shown
for both a high and low pressure case. One skilled in the art will
understand that the pressure within a particular column will vary
slightly between the bottom and the top of the column. The listed
pressure in Table 3 is the nominal pressure, representing, for
example, the pressure at the feed inlet to the column. Pressures at
various points along the column will vary from the nominal value
depending on such factors as, for example, the size of the column,
the type and number of internals in the column, the throughput
through the column, and the magnitude of the nominal pressure.
These features are part of standard fractionation column design,
and the methods for accounting for these features are well known in
the art. TABLE-US-00003 TABLE 3 Typical Nominal Operating
Conditions for the Fractionation Columns Shown in FIG. 2 Low
Pressure High Pressure Temp .degree. F. Press. Psia Temp .degree.
F. Press. Psia Deethanizer (120) 60 3 120 8 Debutanizer (180) -50 3
0 8 Depropanizer (230) -100 3 -50 12
[0027] LPG may be maintained stored at essentially atmospheric
pressure as a liquid in a storage tank by continuously vaporizing a
small amount of the cold liquid in the tank containing the LPG.
This boil off gas is usually recovered and blended into the fuel
gas stream or recondensed and returned to the storage tank. Because
of its value in the system, it is desirable to recondense this boil
off gas and return it to the storage tanks. One method for
recondensing the boil off gas is illustrated in the separate
embodiment shown in FIG. 3.
[0028] The embodiment shown in FIG. 3 is essentially the same as
the scheme illustrated in FIG. 2 except storage tanks (22) and (24)
for holding the C.sub.3 and C.sub.4 products, respectively, have
been added to illustrate the boil off gas recovery system.
According to this embodiment, the boil off gas from liquid C.sub.3
product storage tank (22) is vented through line 402; likewise the
boil off gas from liquid C.sub.4 product storage tank (24) is
vented through line 432. In the embodiment illustrated in FIG. 3,
the two vented vapor streams (402) and (432) are pressurized
slightly through blowers (430) and (440), respectively, and then
combined for passage to the 3.sup.rd fractionation zone (230) also
referred to as the depropanizer. The boil off gases are then
recovered as liquid products from the fractionation zone and
returned to the respective tanks (22) and (24).
[0029] FIG. 3 illustrates the combination of the C.sub.3 boil off
gas (402) being combined with the C.sub.4 boil off gas (432) and
the combination passed to distillation for recover of separate LPG
streams. Alternatively, the C.sub.3 boil off gas (402) and the
C.sub.4 boil off gas (432) may be passed separately to the
depropanizer. In this embodiment separate liquid C.sub.3 products
and liquid C.sub.4 products are recovered from the fractionation
zone as before.
* * * * *